This invention pertains to methods of making multimode silica-based optical fibers, and to fibers made by the method.
Multimode (MM) silica-based optical fiber is well known. Briefly, such fiber has a core that is contactingly surrounded by a cladding. The core has an effective refractive index greater than the refractive index of the cladding. The core radius and the refractive indices of core and cladding material are selected such that the optical fiber supports two or more (typically hundred or more) guided modes at an operating wavelength, e.g., 0.85 or 1.31 xcexcm. Guided modes are conventionally designated LPvxcexc, where the azimuthal mode number v is an integer greater than or equal to zero, and the radial mode number xcexc is an integer equal to or greater than 1. LP01 is the fundamental mode, and all other modes are higher order modes. The total number N of guided modes supported by a given MM fiber is approximately equal to V2/2, where V is the normalized frequency parameter (V-number) of the fiber.
It is well known that a MM fiber with appropriately graded core refractive index can have substantially greater bandwidth than a similar MM fiber with step index profile. In particular, a conventional core refractive index profile is expressed by                     (                              n            ⁡                          (              r              )                                -                      n            clad                          )                    n        ⁡                  (          r          )                      =          Δ      ⁢              xe2x80x83            ⁢              (                  1          -                                    (                              r                /                                  r                  core                                            )                        α                          )              ,
where r is the radial coordinate , rcore is the core radius, nclad is the refractive index of the cladding adjacent to the core, xcex94 is the normalized refractive index difference between the center of the core and the cladding (with correction for undesired index dip), and xcex1 is a free parameter. For xcex1=2, the profile is referred to as a parabolic one. The optimal choice of xcex1 and xcex94 depend inter alia on the properties of the materials that make up the optically active portion of the MM fiber and on the intended application. Frequently, xcex1 is about 2.
Efforts have been made to develop fiber index profiles that yield high bandwidth to equalize the transit times of high order modes in a multi-mode fiber and to compensate for the center dip. See for instance, K. Okamoto et al., IEEE Trans. Microwave Theory and Techniques, Vol. MTT-25, No. 3 (March 1977), at p. 213, and M. Geshiro et al., IEEE Trans. Microwave Theory and Techniques, Vol. MTT-26(2), 1978, p. 115.
During the early days of optical fiber, many patents that pertain to mode coupling in MM fibers and/or to methods of enhancing such mode coupling were issued. For instance, U.S. Pat. No. 3,909,110 discloses step index MM fiber waveguides with intentional fluctuations of the core refractive index. U.S. Pat. No. 3,912,478 discloses introduction of geometrical variations into the fiber by means of gas streams directed against the fiber as it is being drawn. U.S. Pat. No. 3,969,016 discloses mode coupling by means of an enveloping outer jacket which is selectively deformed. U.S. Pat. No. 3,980,459 discloses insertion of a glass rod into the preform during deposition of the core material, resulting in fiber having a longitudinally eccentric index inhomogeneity. U.S. Pat. No. 3,982,916 discloses a preform manufacturing process that involves asymmetric heating to produce circumferentially alternating deposits of doped and undoped glass, the resulting fibers having longitudinal, eccentric, azimuthal index inhomogeneities.
U.S. Pat. No. 4,017,288 discloses a technique for producing optical fibers with longitudinal variation in index of refraction. U.S. Pat. No. 4,028,081 discloses a helical optical fiber loosely confined in a protective sheath. U.S. Pat. No. 4,038,062 discloses a MM optical fiber with reduced modal dispersion as a result of enhanced mode coupling, achieved by means of one or more modulated heat sources. U.S. Pat. No. 4,049,413 discloses a method for producing optical fibers with diameter variations in the core but with uniform overall diameter. The method involves etching of grooves into the preform. U.S. Pat. No. 4,093,343 discloses optical fiber with deliberately induced intermodal coupling, with longitudinally varying perturbations in the fiber. U.S. Pat. No. 4,176,911 discloses a MM optical fiber having a graded profile region followed by an abrupt drop in index, following by a region of constant index. At predetermined intervals the fiber is modified to have a conventional graded index profile.
Co-assigned U.S. patent application Ser. No. 09/326,960, filed Jun. 7, 1999 by S. E. Golowich et al for xe2x80x9cMulti-Mode Optical Fiber Having Improved Refractive Index Profile and Devices Comprising Samexe2x80x9d,now U.S. Pat. No. 6,292,612, discloses MM fiber having a refractive index profile that differs from a conventional xcex1-type profile by at least one of i) a step formed in the index profile at the core/cladding boundary, in conjunction with a linear correction; (ii) a ripple near the core/cladding boundary, in combination with a linear correction, with or without an index step; and iii) an annular ridge formed in the index profile with a center dip.
Thus, the art knows techniques that may yield MM fiber with significant mode coupling and thus with relatively high bandwidth. However, there is still a need for techniques that are effective for increasing maximum bandwidth and for increasing the yield of fiber of average bandwidth, that are manufactureable and can be easily incorporated into currently used fiber manufacturing processes. This application discloses such techniques. All herein cited references are incorporated herein by reference.
It is known from the theory of MM fibers that, if a mechanism exists that thoroughly mixes the modes within a given mode group, and also thoroughly mixes the mode groups, then high bandwidth can be realized without careful grading of the core refractive index. See, for instance, R. Olshansky, Applied Optics, Vol. 14(4), April 1975, p.935. All the modes of a mode group have the same propagation constant xcex2, and different mode groups have different propagation constants.
The above theoretical prediction has been confirmed in plastic MM optical fibers, where high bandwidths (e.g., xcx9c5 GHz km) have been measured on fibers with non-optimal grading of the core index.
It will be appreciated that plastic fiber inherently has severe mode mixing. On the other hand, measurements of silica-based MM optical fibers show that in conventional silica-based fibers relatively little mixing occurs within mode groups and between mode groups.
xe2x80x9cChiralityxe2x80x9d and related terms such as xe2x80x9cchiral structurexe2x80x9d are used herein in the conventional sense, as referring respectively to xe2x80x9chandednessxe2x80x9d and xe2x80x9chanded structurexe2x80x9d.
xe2x80x9cPreformxe2x80x9d herein can refer to the preform tube before collapse as well as the shaped or unshaped preform rod after collapse. The meaning will be clear from the context.
Broadly speaking, the invention is embodied in a method of making silica-based MM optical fiber having high bandwidth, typically greater than 100 MHz km.
More specifically, the invention is embodied in a method of making a silica-based MM optical fiber having a core and a cladding that contactingly surrounds the core, the core having a radially varying refractive index. The method comprises providing a silica-based optical fiber preform, and drawing the optical fiber from the preform. Significantly, at least a portion of the preform has a non-circular cross section, and the drawing step comprises drawing the fiber from the preform such that the drawn fiber has a chiral structure. Associated with the chiral structure typically is a repeat length or period. The repeat length typically is 10cm or less, and typically varies along the length of the fiber.
In a preferred embodiment, the non-circular preform rod with non-circular core is formed by collapsing the tubular preform while maintaining a reduced pressure in the tube. In another embodiment, a non-circular core is formed by selective removal of glass (e.g., by grinding or by plasma etching) from the outside of the preform rod, followed by fiber drawing at relatively high temperature such that the resulting fiber has substantially circular cross-section, but with a non-circular core.
In either case, a chiral structure is introduced into the fiber during drawing, generally by twisting of the fiber relative to the preform, or by twisting of the preform relative to the fiber. In a preferred embodiment the fiber is twisted alternately clockwise and counterclockwise with respect to the preform, substantially as disclosed in U.S. Pat. No. 5,298,047.
The presence of a non-circular core, preferably together with the chirality introduced during fiber draw, typically results in significant mode coupling and, consequently, in high bandwidth of the fiber, as well as in reduction of the sensitivity of the bandwidth to the details of the index profile. In a further exemplary embodiment of the invention the index profile differs from a conventional parabolic or near-parabolic one, and is selected such that, in combination with the non-circular core and the imposed chirality, the mode mixing and consequent high bandwidth are increased. Exemplarily, the index profile is as disclosed in the above referenced U.S. Pat. No. 6,292,612. The increase in bandwidth can be realized using either an overfilled mode launch or with a restricted mode launch or both.
The invention is also embodied in an article that comprises silica-based MM optical fiber having non-circular cross section and chirality sufficient to cause mode coupling between modes in a given mode group, and between mode groups of the MM fiber, such that the MM fiber has large bandwidth. Exemplarily, the article is an optical fiber communication system, e.g., an optical fiber local area network (LAN).